Typical wireless communication and power transfer schemes in use today employ inductive coupling between a resonant primary coil antenna and a resonant secondary coil antenna. For example, wireless communication includes near field communications (NFC) related functions or Europay, MasterCard and Visa (EMVCo) based contactless payment and wireless charging. A common issue seen in these systems is a non-uniform nature of induced voltage in the secondary coil antenna as it is moved with various x-y offsets at a fixed distance z from the primary coil antenna. Moreover, the nature of the non-uniformity is a function of the size of the secondary coil antenna relative to the primary coil antenna.
It is noted that the above non-uniformity may be particularly pronounced when the primary and the secondary coil antennas are identical in size and geometry. This configuration leads to a high degree of coupling between the coils antennas, leading to a splitting of modes (in the frequency domain), where the resonant frequencies of the combined coupling coil antennas are different from the resonant frequency of the individual coil antenna, which happens to be the driving frequency of signal in the primary coil antenna.
The non-uniformity further results in an unpredictable user experience and is best exemplified in contactless credit card reading performance. For example, the contactless credit card may have coil strips or coil antenna that cover a) all area of the card, b) half area of the card, or c) quarter area of the card. In addition, the center of the embedded coil strips in the credit card typically does not coincide with a geometric center of the credit card making it hard for the user to find the best location to tap at a reader and elicit a response.
The user's difficulty is also reflected in the challenge faced in passing EMVCo Contactless Payment Certification by reading a diverse set of representative cards at various orientations and positions. Failing to read the cards at any of the defined positions/orientations results in a certification failure. Hence, the certification is gated by the performance at the worst location in the operating volume. Therefore, having a very high field at one point, followed by a point with a weak field nearby is undesirable.
In wireless charging scenario, there is also a need to carefully align a wireless charging device on the charging mat so as to maximize the power transfer. Therefore, there is a need for a design to provide the same level of coupling over a wide x-y area for a given coupling secondary coil antenna.